Question: Brandon is 3 times as old as Gabriela. Twenty years ago, Brandon was 8 times as old as Gabriela. How old is Gabriela now?
Explanation: We can use the given information to write down two equations that describe the ages of Brandon and Gabriela. Let Brandon's current age be $b$ and Gabriela's current age be $g$ The information in the first sentence can be expressed in the following equation: $b = 3g$ Twenty years ago, Brandon was $b - 20$ years old, and Gabriela was $g - 20$ years old. The information in the second sentence can be expressed in the following equation: $b - 20 = 8(g - 20)$ Now we have two independent equations, and we can solve for our two unknowns. Because we are looking for $g$ , it might be easiest to use our first equation for $b$ and substitute it into our second equation. Our first equation is: $b = 3g$ . Substituting this into our second equation, we get: $3g$ $-$ $20 = 8(g - 20)$ which combines the information about $g$ from both of our original equations. Simplifying the right side of this equation, we get: $3 g - 20 = 8 g - 160$ Solving for $g$ , we get: $5 g = 140.$ $g = 28$.